The present invention relates to an optical scanning system using a laser diode array.
As is well known, a laser diode array is used as a light source in which a plurality of semiconductor laser diodes are integrally formed so that light-emitting portions thereof are array-arranged. Recently, a scanning method has been proposed in which a plurality of image lines on a surface of a recording medium are scanned at one time by simultaneously deflecting a plurality of luminous fluxes (beams) emitted from the above-mentioned light source. By this scanning method, it becomes substantially possible to obtain an increased scanning speed.
FIG. 1 is a view showing an example of a laser diode array. An illustrated laser diode array LDA is made up of two light-emitting portions spaced at a distance L. The light-emitting portions emit elliptical beams B1 and B2 each having the major axis in a direction perpendicular to a heterodyne interface 9. Each of the elliptical beams B1 and B2 has a divergence of an approximately 30.degree. width in the major axis for the energy mesial magnitude, and a divergence of an approximately 10.degree. width in a minor axis for the energy mesial magnitude. The minor-axis is a direction parallel to the heterodyne interface 9. Generally, there is a limitation of approximately 0.1 mm on the distance L between the light-emitting portions.
FIG. 2 is a view illustrating a state where two beams emitted from the laser diode array LDA are focused so as to form spots SP1 and SP2 on a surface to be scanned. By deflecting two beams at the same time, it becomes possible to simultaneously scan lines ln.sub.1 and ln.sub.2 by the spots SP1 and SP2, respectively. As described before, the distance L between the light-emitting portions has a limitation of approximately 0.1 mm, and it is impossible to obtain the distance L shorter than the above limitation. When the distance L is actually set equal to an interval Ps between the lines ln.sub.1 and ln.sub.2, the interval Ps becomes too large. For this reason, a method as shown in FIG. 3 has been proposed in which the heterodyne interface 9 of the laser diode array LDA is inclined at a fine angle .theta. with respect to a main scanning direction MS so that a distance P.sub.LS becomes equal to the line interval Ps shown in FIG. 2. The above is disclosed in a Japanese Laid-Open Patent Application No. 56-69611, for example. Therefore, the spots SP1 and SP2 are spaced at a distance P.sub.M in the main scanning direction MS, corresponding to a length of L cos .theta..
FIG. 4 is a view which simply shows a conventional optical scanning system. FIG. 4(A) shows a state obtained by expanding an optical path extending from a light source 10 to the scanned surface 8, to a single plane. The up/down direction of FIG. 4(A) corresponds to the main scanning direction MS in the scanned plane. Hereinafter, the up/down direction in FIG. 4(A) is simply referred to as the main scanning direction. FIG. 4(A) also shows a state of beams on a plane which the beams deflected by a deflection/reflection surface 4 of a deflector scan, or "a deflector plane". On the other hand, FIG. 4(B) shows a state obtained by expanding the conventional optical scanning system to a single plane along the above-mentioned optical path. The plane of FIG. 4(B) is a plane which includes the optical path and is perpendicular to the above-mentioned deflection plane. Hereinafter, this plane is simply referred to as "a plane perpendicular to the deflection plane". In FIG. 4(B) showing a deflection plane, the up/down direction corresponds to a sub scanning direction. Hereinafter, this up/down direction is referred to as a sub scanning direction.
Divergent beams emitted from the light source 10 are altered to parallel luminous fluxes by a collimator part 11, and then, due to the function of a cylindrical lens 12, form respective line images in the main scanning direction in the vicinity of the deflection/reflection surface 4 in the plane perpendicular to the deflection plane. An imaging lens 13 is an anamorphic lens, and focuses the parallel luminous fluxes on the scanned surface 8 in the main scanning direction. On the other hand, in the sub scanning direction, or in other words, the plane perpendicular to the deflection plane, the imaging lens 13 couples the position of the deflection/reflection surface 4 with the scanned surface 8 so that a nearly conjugate relationship is established therebetween. As a result, each beam is imaged in the shape of a spot on the scanned surface 8.
However, the conventional scanning system having the laser diode array LDA has the following disadvantages.
As described previously, the divergent angle of each beam emitted from the laser diode array LDA is large in the direction perpendicular to the heterodyne interface 9. Therefore, by collimating the two beams B1 and B2 by the collimator part 11, the flux diameter of each collimated beam becomes large in the direction perpendicular to the heterodyne interface 9 in cases except that the numerical aperture of the collimator part 11 is extremely small.
As described previously, the laser diode array LDA is arranged in such a manner that the heterodyne interface 9 thereof is kept inclined at an angle .theta. with respect to the main scanning direction. In this case, the angle .theta. is set very small. Therefore, each of the collimated beams is thin in the main scanning direction, and is thick in the sub scanning direction. As a result, when the above-mentioned beams are imaged on the scanned surface 8 by using the optical scanning system of FIG. 4, it is difficult to obtain a desired shape of each imaged spot such as the spots SP1 and SP2 shown in FIG. 2 in which it is slightly longer in the sub scanning direction than in the main scanning direction. That is, although the width of an imaged spot in the sub scanning direction is adjustable by using the cylindrical lens 12, the width thereof in the main scanning direction is necessarily determined by the collimator part 11.
As a result, it is necessary to design the collimator part 11, depending on the width of an imaged spot in the main scanning direction. In cases when a desired spot width is not obtained in the assembled optical scanning system, there is no way except for an exchange of the collimator part 11 in order to obtain a desired spot shape. This increases cost.